This study (which is in its fourth of five years, and for which we are requesting four additional years of support) is designed to address and resolve as-yet unanswered questions about the effects and effectiveness of a lower-than-standard range of lithium (0.4-0.6 mEq/1) in the maintenance treatment of patients with bipolar affective illness. We are comparing and contrasting this lower level with a more standard one (0.8-1.0 mEq/1) regarding protection against relapse, the presence of less severe affective symptoms, and association with unwanted and toxic effects. This double-blind study is being run concomitantly at both the Massachusetts General Hospital and the Long Island Jewish-Hillside Medicare Center. Projected intake to completion is 175 subjects at the two sites, with patients randomly assigned to the two different treatment regimens.